True mil-spec AR-15 low shelf/M16 cut lower receivers. Many if not most companies advertise as, "true mil-spec", but it is nothing more than an advertising phrase. They are not internally machined to the same dimensions. ATG Defense XG-15 receivers are 100% military specification machined. Internal pocket is cut to a standard M-16/M-4 receiver. Buy with assurance that ATG Defense' guarantee is satisfaction and customer service. For Texans, by Texans. ***10% off First Responders and Current and Former Military.*** Any purchase of an ATG Lower comes with $200 dollars off of building it into a full rifle sometime in the future if desired.